Windows Server Virtualization and memory

Mark Wilson asked an interesting question about virtualization. Would Windows Server Virtualization provide the ability to overcommit resources, therefore increasing consolidation ratios.

A few new bits of information have come out since my last Virtualization round-up and two of them were about memory.

Page Sharing. : a feature which identifies identical pages in memory and shares them among multiple virtual machines, regardless of the guest OS. If 10 VMs are running different OSes they will not have many pages in common. 10 copies of the same OS will have a lot more overlap.

Memory Reserves. think of it as a minimal memory guarantee for a virtual machine. Configuring memory for a virtual machine you (a) Allocate memory (b) Assign a (percentage) reserve 
Virtual Server always reserves 100%. Windows Server Virtualization defaults to 100% - which ensures the best performance out of the box. But if you reserved only 75% of an allocation of  2 GB might be paged to disk.

There's no free lunch here. You need to be very careful about using reserves under 100% - you can pack more VMs into one box, but the performance hit when the host OS needs to page could be severe. My hunch is for most workloads, telling a VM it has 2GB and letting it manage it's own paging will be better than telling it has 4GB and having the host OS manage which 2GB of that is RAM at any moment.

Technorati tags: Microsoft, Longhorn, Virtualization